aws-s3 can't find xml-simple, but in gem list

Question

I'm transitioning to heroku, and need to have AWS-s3 connections to deal with a variety of graphics.

I've installed the aws-s3 gem, but one of its dependencies is not being found: xml-simple. My belief is that this is a standard part of RoR, and it is in the gem list. When I deploy to heroku, all is fine, but on my development server, it isn't being found when the code uses it to check the existence of a graphic. It works fine from the console, using s3sh.

I'm not quite sure why this is -- what do I need to check? Using OS X 10.6, on a 64 bit machine -- can this be part of it?